Why have some national governments acted more aggressively to address the HIV/ AIDS pandemic than others? More specifically, what explains, widely varied responses across Brazil and South Africa—two countries where one might have expected more similarity than difference? We argue that boundary institutions—those sets of rules and practices that give social and political meaning to group identities—help explain this puzzle. Institutions interact with other pressures to structure the dissemination of information, the construction of risk, and priorities within society. Where institutions divide groups deeply, elites and ordinary citizens are less likely to feel volnerable, and more likely to blame other groups, making aggresive government action far less likely.